The purpose of this investigation is to characterize the relative physiological roles and evolutionary origins of the aldehyde and xanthine oxidases of mammalian liver. This is a particularly puzzling problem because the great diversity in kinetic properties among the liver aldehyde oxidases is superimposed on a background of gross structural similarities both within the group of aldehyde oxidases and between that group and xanthine oxidase. At issue are the following questions: Are the species specific properties of aldehyde oxidase related to specific metabolic roles and needs? Or, are the differences in the properties of aldehyde oxidases among mammals an indication of an enzyme in the process of evolution? What is the biological significance of the great enzymological similarities in xanthine oxidase and aldehyde oxidase? Could these enzymes have common genetic origins? The method of affinity chromatography will be used to isolate aldehyde and xanthine oxidases in a form as close to the native one as possible. Exhaustive kinetic analyses will be conducted as a device for defining similarities and differences among the individual proteins. The nature of the subunit structure will be investigated using both genetic and biochemical methodologies. Similarities and diversity in the structures of subunits will be assessed by metal, flavin amino acid analyses and peptide finger printing. Genetic and biochemical procedures will be used to evaluate the possibility of the interchangeability of subunits among the aldehyde oxidases and with xanthine oxidase. In the ultimate, sequencing of selected enzymes and or subunits will be carried out.